GOP most likely picks up 7-8 seats (SD, WV, MT, AR, LA, AK, NC) possibly IA with 52-53 seats in 2015 going by the current environment right now. Then 2016 comes into focus if they win 52 seats then Dems can at least breathe easy that they would only need three seats to take it back (WI, IL, PA) already there for them but 53 seats or more for the Rs its a steeper climb for the Ds.
The Democrats could potentially pick up Senate seats in Florida, New Hampshire, Ohio, North Carolina, Missouri (if Jay Nixon runs), Iowa (if Chuck Grassley retires) and Arizona (if John McCain retires). If those seats and the ones you mentioned all go to the Democrats, then they would have a 57 or 58 seat majority after 2016.